1. Field of the Invention
The present invention relates to an optic signals processor, notably a time integrating correlator, comprising a charge-coupled device or CCD.
It is known that CCDs can be used to carry out the optic processing of a signal (i.e. after this signal has been converted from an electrical signal into an optic signal), especially in two-dimensional processors that perform real-time optic processing of signals such as those delivered by radar receivers or telecommunications receivers.
In this context, it must be specified that although the following description relates to an application of the invention to a two-dimensional time integration correlator, this example is used purely as an illustration: the present invention is quite broader in its scope than the particular example itself.
More precisely, the present invention can be applied to any optic processor of data applied to the detection of a beam by a CCD, whether one-dimensional (CCD linear array) or two-dimensional array, provided that this processor has to perform the term-by-term subtraction of two series of data, for example, the subtraction of two vectors (in the case of a one-dimensional array) or of the lines or columns of two matrices (in the case of a two-dimensional array).
2. Description of the Prior Art
The principle of the optic processing of signals is known. According to this principle, when the bandwidths exceed values permissible in electronics, the electrical signal to be analyzed is converted into a modulated beam. This modulation may be done either by the direct modulation of a source (typically a laser diode) or by the indirect modulation of a continuously emitting laser source using, for example, an opto-electronic component.
Then, using deflector means such as acousto-optic means, the modulated beam is made to scan in one or two directions corresponding to the dimensions of the correlation space.
Various configurations of optic processors using this technique are described in an article by P. V. Gatenby and R. J. Sadler, Acousto-Optic Signal Processing in GEC Journal of Research, Vol. 2, No. 2, 1984, pp. 88 to 95, which may be consulted for fuller details.
The basic configuration of an optic processor such as this is shown schematically in FIG. 1 of the appended drawings, in an example corresponding to the optic processing of the signal s.sub.n (t) coming from a radar receiver, in order to determine the ambiguity function by time integration in the space D/f.sub.D, i.e. the distance/speed space (the speed being represented by the Doppler frequency).
In this processor, a first beam from a laser source S is modulated at 1 by the signal s.sub.n (t) coming from the radar receiver. The modulated beam produced is deflected in the horizontal direction (with respect to the convention of the drawing) by an acousto-optic modulator 2 controlled by a sampled signal p(f.sub.i) corresponding to the N samples of the distance-domain for the signal s.sub.n (t).
A second signal is deflected in the perpendicular direction by a second acousto-optic modulator 3, controlled by a signal r.sub.m (t), which is also sampled, corresponding to M Doppler ports of the signal s.sub.n (t).
The two resultant beams then strike a charge-coupled device 4 (shown separately in a plane view, and then in detail, in FIG. 2) formed by an array 10 of M lines 11 of N cells 12 each. Only one of these lines has been shown in FIG. 2.
It is known that that each exposed pixel of an image zone (referenced ZI in FIG. 2) of a charge-coupled device picks up an incident light flux and converts the corresponding energy into an electrical charge. This electrical charge is stored at the location of the pixel in an electrical capacitor and gets increased throughout a period of exposure, known as the "integration time".
The resultant charges are then transferred from one point to the next one in the array until (either directly as shown in FIG. 2 or through a non-photoactive buffer zone called a "memory zone"), they reach a component 40 capable of detecting each stored charge and of converting it into a voltage or current that can be used by the processing circuits 5 placed downline.
The result of the processing operation performed by the circuits 5 will be the ambiguity function, shown at 6, making it possible to determine the position of the target tracked by the radar in the domain (distance and velocity).
One of the difficulties encountered in this processing operation is related to the fact that, when optic methods are used, it is a light energy and no longer a simple signal voltage that will be integrated. As a result, the charge produced in each cell by the charge-coupled device will take the form of a quadratic sum of two terms (which shall be described in greater detail here below) that therefore break down into a sum of two squared terms and one product term.
The product term of this quadratic sum constitutes the useful signal of the correlation, while the sum of the two squared terms constitutes the mean component of the base level or bias, which gets added on to the useful signal and to the correlation pedestal.
The presence of this bias component creates a twofold difficulty:
First of all, it increases the absolute value of the resultant signal, since the useful signal is increased by the bias component, which is far greater than the weakest signals.
The consequence of this will be a notable increase in the dynamic range of integration, for it will be necessary to integrate the total signal to subsequently recover only the useful signal therefrom at output.
It will be noted, in this context, that the increase in the dynamic range comes from the fact that the procedure uses optic means, integrating no longer a voltage but a signal power (quadratic detection), thus doubling the necessary dynamic range.
Secondly, it is not possible to predict the value that should be deducted from the result to extract the useful signal: the level of the bias component is not constant for, as shall be explained further below, it depends on (among other factors) the mean value of the signal.
A special processing to suppress this bias must therefore be provided for.
It will be noted, incidentally, that this term "bias" will generally include the continuous component (which, for its part, is constant in principle) added on to the modulating signal during the modulation so that the negative components of the useful signal can be processed, in shifting this useful signal towards the positive values.
The remedy proposed in the prior art consists in using two components that are phase-shifted by radians with respect to each other, in simultaneously carrying out two correlations in two identical charge-coupled devices, and in taking the difference between the samples resulting from these two correlations, so as to thus extract only the useful signal therefrom.
For, the phase-shifting of one of the modulating signals will change the sign of the above-mentioned product term, hence the sign of the useful component of the signal but not the sign of the two squared terms (owing to the squaring operation). The subtraction of the two resulting samples will enable the elimination of these squared terms, only the useful component of the signal being preserved.
The theoretical aspects of this processing are, for example, developed in an article by M. W. Casseday, N. J. Berg, I. J. Abramovitz and J. N. Lee, Wide-Band Signal Processing Using The Two-Beam Surface Acoustic Wave Acousto-Optic Time Integrating Correlator, in the IEEE Transactions On Sonics And Ultrasonics, Vol. SU-28, No. 3, May 1981, pp. 205 to 212, which may be consulted for fuller details.
In practice, a configuration such as the one illustrated in FIG. 1 is used by splitting the optic beam, generated by the optic source S, into two and also by splitting the beam at output of the first acousto-optic modulator into two, by means of a beam separator, and by modulating the second branch of the input beam by an acousto-optic component 3' that is similar to the component 3, but is controlled by a signal r.sub.m *(t) phase-shifted by .pi. in relation to the signal r.sub.m (t).
The resultant beam produced by this second branch strikes a second charge-coupled device 4', identical to the charge-coupled device 4.
One of the two signals coming from the respective charge-coupled devices 4 and 4' is then subtracted from the other one by a circuit 7 enabling the suppression of the bias, before these two signals are applied to the processing circuit 5.
However, while this approach is satisfactory in theory, it has two series of practical drawbacks.
First of all, it makes it necessary to split the beam into two and, hence, to duplicate the optic components, with the numerous difficulties entailed by duplication, notably:
the increase in the cost, due to the duplication of the components, the addition of a beam separator upline and of a processing circuit downline, to obtain the difference between the signals,
the necessary correction of the relative dispersals of the components of the two branches (notably that of the charge-coupled devices),
the optic problems of alignment of the beams, notably if it is desired to provide for efficient
the drastic need to avoid optic or electrical saturation in one of the branches, in order to have no loss of signal;
the major degradation of the signal-to-noise ratio owing to the additional noise factor introduced by the additional active components added to the chain.
The second series of drawbacks relates to the fact that the prior art approach provides no remedy to the loss of dynamic range introduced by the bias, which is suppressed only downline of the charge-coupled devices.
A major part of the dynamic range is lost because it is necessary to integrate not only the useful signal proper to be correlated (or to be processed in another way) but also the correlation pedestal component: this correlation pedestal component will be subsequently suppressed but its inherent level is already of the order of 70 dB. Furthermore, it is necessary to be in a zone of operation that is not subject to compression of the level, otherwise suppression of the bias will be made impossible.
Besides, the best CCDs available at present, for example those marketed under the brand name of Dynasensor by Dalsa Inc., only have a dynamic range of the order of 120 dB. The limits of this dynamic range are essentially dictated by the risk of saturation of each pixel under the effect of an excessively prolonged illumination (by the effect of overflow on to the neighboring pixels) and above all of saturation of the electrical reading amplifier of the detection circuit (a dynamic range of 120 dB corresponds to a range of voltage values that may go from 10 nV to 10V, which amounts to a considerable voltage difference). In many applications, however, this 120 dB limit is still insufficient for some processing operations or for some measurements to be made.
It would therefore desirable to have a dynamic range that is notably greater than 120 dB, even if what is to be used is only the upper 120 dB of the range, which contain the useful zone of the signal after suppression of the bias component as well as of the correlation pedestal.
It is true that, to this end, the dynamic range could be increased through the use of two distinct charge-coupled devices: the first one works like a standard CCD and the second one, placed downline with respect to the first one, is not photoactive but is used to carry out a second integration while the basic integration continues in the first component.
However, there would be major deterioration in the signal-to-noise ratio, for the use of two separate components necessarily dictates a dual signal conversion (the conversion of the charge into a voltage or a current, to come out of the first CCD array, then the conversion of this voltage or current into a charge, to enter the second CCD array): this constraint entails heavy penalties owing to the noise factor introduced by the active components carrying out these conversions.
Furthermore, this noise will be greatly increased by the number of inter-CCD transfers that will be needed to obtain the result.
Finally, such a technique would entail a major increase in the total integration time, owing to the charge transfer time, which is of the order of one microsecond per sample: this would give a total time of four seconds for each transfer in an array of 2000 .times. 2000 pixels for example (in the case of a single output) or two milliseconds in the case of 2000 parallel outputs.